In a conventional wireless packet communication apparatus, only one radio channel to be used is determined in advance and it is detected whether the radio channel is idle or not (carrier sense) prior to transmitting data packets. Only when the radio channel is idle, only one data packet is transmitted. By such a control, one radio channel can be used together at different times by a plurality of STAs ((1), International Standard ISO/IEC 8802-11 ANSI/EEE Std. 802.11, 1999 edition, Information technology—Telecommunications and information exchange between Systems—local and metropolitan area networks—Specific requirements—part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications; (2) “Low-powered Data Communication System/Broadband Mobile Access Communication System (CSMA) Standard, ARIB STD-T71 version 1.0, Association of Radio Industries and Businesses, settled in 2000).
In such a wireless packet communication apparatus, in order to improve the maximum throughput, for example, there provided a method in which the frequency band per one radio channel is expanded to speed up the transmission rate in the PHY layer.
However, as pointed out even in a document (Iizuka et al., “5 GHz Wireless LAN System Based on the IEEE802.11a standard—Packet Transmission Characteristics—”, B-5-124, Proceedings of the Electronics Information and Communication Engineers, Society Conference 2000, September 2000), for collision of packets avoidance, a constant transmission deferral duration, which does not rely on the transmission rate in the PHY layer immediately after packet is transmitted, needs to be provided. With the transmission deferral duration provided, as the transmission rate in the PHY layer increases, the transmission efficiency (the ratio of the transmission rate in the PHY layer to the maximum throughput) of data packets decreases. Therefore, although the transmission rate in the PHY layer increases, throughput is not improved significantly.
Correspondingly, as a method of improving the maximum throughput without expanding frequency band per one radio channel, application of a MIMO technique (Kurosaki et al., “100 Mbit/s SDM-COFDM over MIMO Channel for Broadband Mobile Communications”, Technical Reports of the Institute of Electronics, Information and Communication Engineers, A•P 2001-96, RCS2001-135(2001-10)) is considered. The MIMO technique is a method in which different data packets are transmitted simultaneously from a plurality of antennas through the same radio channel and the plurality of data packets transmitted simultaneously are received through the same radio channel by digital signal processing corresponding to the difference in propagation coefficients of the respective data packets received in a plurality of antennas of the opposite STA. Further, according to propagation coefficient or the like, a MIMO number is determined.
Meanwhile, when the respective ones of STAs have a plurality of Wireless Network Interfaces and a plurality of radio channels are available, different radio channels are used respectively among a plurality of STAs, which can improve throughput compared to the case where one radio channel is time-divided to communicate. For example, by using a radio channel CH1 between the STAs A and B and by using a radio channel CH2 between the STAs A and C, respective data packets can be transmitted and received simultaneously between the STA A and the STAs B and C, as shown in FIG. 17. Alternately, by using the radio channels CH1 and CH2 between the STA A and the STA B, two data packets can be transmitted and received simultaneously, as shown in FIG. 18.
However, in the case where center frequencies of a plurality of radio channels to be used simultaneously come close to each other, an effect of leakage power leaking into the frequency band which is used between one radio channel and the other radio channel becomes great. In general, when data packets are transmitted, a transmit-side station first transmits data packets and then, a receive-side station returns an acknowledgment packet (ACK packet or NACK packet) for the received data packet to the transmit-side station. When the transmit-side station tries to receive the acknowledgement packet, an effect of leakage power from other radio channels where the packets are transmitted simultaneously becomes a problem.
For example, as shown in FIG. 19, it is assumed that the center frequencies of the radio channel CH1 and the radio channel CH2 come close to each other and transmission times for packets to be simultaneously transmitted from the respective radio channels are different. Here, the data packet transmitted from the radio channel CH1 is short. Therefore, when an ACK packet for the data packet is received, the radio channel CH2 is in transmission. For this reason, in the radio channel CH1, it is likely that an ACK packet cannot be received due to leakage power from the radio channel CH2. Under such a condition, even though transmissions are performed simultaneously by use of a plurality of radio channels, throughput can not be expected to be improved.
Further, when the transmission rates in the respective radio channels are equal to each other, such a case occurs due to the difference in the data sizes of the respective data packets. In addition, when the transmission rates of the respective radio channels are not equal to each other, such a case occurs due to the difference in (the data sizes/the transmission rates) of the respective data packets. That is, such a case occurs due to the difference in the packet time lengths which are transmission times for the respective data packets.
However, in a wireless LAN system or the like, the data sizes of data frames to be input from a network are not constant. Accordingly, when data frames to be input are sequentially converted into data packets to be transmitted, the packet time lengths (transmission times) of the respective data packets also change. For this reason, although a plurality of data packets are transmitted simultaneously as shown in FIG. 19, the difference in the packet time lengths of the respective data packets occurs, so that it is very likely to fail to receive an ACK packet.
Subsequently, the respective data frames on a buffer are divided at even intervals into a plurality of data blocks, so that data packets are generated from the respective data blocks. Accordingly, a plurality of data packets having the same sizes are acquired. Therefore, if these packets are transmitted simultaneously through a plurality of radio channels, an effect of leakage power can be avoided. However, in this case, since the respective data frames input into a buffer are divided into a plurality of data blocks, the sizes of the respective data packets becomes smaller than usual, so that effective throughput decreases.
In addition, for example, with a predetermined number of data frames provided on a buffer, a plurality of data frames having the same sizes are extracted among those data frames. If the extracted data frames are converted into data packets and controlled so as to be transmitted simultaneously, an effect of leakage power can be avoided. However, in this case, since transmissions have to be delayed until a plurality of data frames having the same sizes are provided on a buffer, the transmissions can not be effectively commenced and reduction in effective throughput can not be avoided. In addition, when data frames having the same sizes do not appear for a long time, a transmission delay is increased.
In addition, when transmissions of data packets are performed between a plurality of receivers through wireless links as shown in FIG. 17, it is assumed that the transmissions of the data packets are performed at transmission rates which are different for the respective receivers. In this case, if data packets which are configured by data frames having the same sizes but have different receiver addresses are transmitted simultaneously, the transmissions are not completed simultaneously and the receptions of acknowledge packets fail due to leakage power. Accordingly, there is a case where data packets whose receiver addresses are different can not be transmitted simultaneously even though they have the same sizes. For this reason, it is very likely that the wait time until a plurality of data packets having the same receiver addresses and the same sizes appear becomes long.
Correspondingly, if the respective STAs to be receivers grasp available transmission rates in advance, packet time lengths (transmission times), which are determined by the data sizes of data packets and the transmission rates to be used, can be grasped. Therefore, a plurality of data packets having the same packet time lengths can be selected simultaneously. When transmissions of a plurality of data packets having the same packet time lengths are commenced simultaneously, the transmissions of them are completed simultaneously. Therefore, acknowledge packets for the respective data packets can be received approximately at the same time and an effect of leakage power can be avoided.
However, in the conventional wireless LAN system defined by IEEE 802.11a, as a transmission rate to be used for transmission of an acknowledge packet, the maximum mandatory rate (any one of 6, 12, and 24 [Mbit/s]) which does not exceed the transmission rate of the received data packet addressed to the own station is selected. For this reason, through radio channels CH1 and CH2 where transmission rates are different with respect to two STAs as shown in FIG. 20, data packets (1) and (2) having the same packet time lengths are transmitted simultaneously and acknowledge packets ACK (1) and ACK (2) corresponding to the respective data packets are received simultaneously. Nevertheless, since transmission times for the respective ACK packets are different, a difference occurs in a time when the receptions of the respective ACK packets are completed. At this moment, right in the middle of reception of ACK (2) through the radio channel CH2, if data packets are transmitted from the radio channel CH1 where the ACK reception is completed previously, it is likely that the reception of ACK (2) through the radio channel CH2 fails.
In other words, there is a case where data packets whose receiver addresses are different cannot be transmitted simultaneously even though the packet time lengths are the same according to the respective transmission rates. For this reason, it is very likely that the wait time until a plurality of data packets having the same receiver addresses and the same sizes appear becomes long.
In addition, as shown in FIG. 21, data packets having different receiver addresses from one transmit-side station are superposed into one radio channel by the MIMO to be transmitted at the same time, the data packets are separated by the respective receiver terminals, and the data packets addressed to the own station can be received. At this time, although the respective receive-side station simultaneously transmits acknowledge packets for the data packets addressed to the own station through the same radio channel, the transmit-side station cannot receive a plurality of acknowledge packets returned simultaneously through the same radio channel.
An object of the present invention is to provide the wireless packet communication method and wireless packet communication apparatus which can reduce probability of reception failure of acknowledgment packets and improve effective throughput by use of a plurality of radio channels and by the MIMO, even though an effect of leakage power appears between STAs when data packets having different receiver addresses are transmitted simultaneously.